1. Field of the Invention
The present invention relates to a pulsating combustion system including a pulsating combustor and to a pulsating combustion system comprising a pair of pulsating combustors parallel-connected.
2. Description of the Related Art
Normally, a pulsating combustion system including a pulsating combustor comprises, in addition to a combustion chamber of the pulsating combustor for burning a mixture gas, i.e., air-fuel mixture, intermittently or pulsatively, a tail pipe for exhausting exhaust gas connected to an exhaust port of the combustion chamber, an aerodynamic valve provided in an air passage for supplying air to the combustion chamber, the aerodynamic valve having its forward flow efficiency larger than backward flow efficiency, an air-intake chamber connected upstream of the aerodynamic valve to reduce noises, a fuel supply valve for limiting a quantity of fuel flowing into the combustion chamber, an ignitor for igniting the mixture gas supplied into the combustion chamber at the time of start, an exhaust chamber connected downstream of the tail pipe, and an air-supplying fan having a small capacity connected upstream of the air-intake chamber.
In the pulsating combustion system of the type as described above, air is fed into the combustion chamber by the air-supplying fan to mix with fuel. When the mixture gas is ignited by the ignitor, it is explosively combusted. As a result, the pressure within the combustion chamber increases, and the combustion gas is exhausted at high speed through the exhaust port of the combustion chamber. As a result of this gas-exhaustion, a negative pressure is generated within the combustion chamber. When the negative pressure is generated within the combustion chamber as described above, air and fuel are self-attracted. When the air and the fuel flow into the chamber, each in a predetermined amount, and are mixed, the resultant mixture is ignited by the flame remaining in the chamber. The explosive combustion is again performed. Therefore, normally, in the pulsating combustion system, the air-supplying fan can be stopped during the normal combustion, which is one of merits.
Recently, however, it has been desired that the pulsating combustion system having a pulsating combustor as described above is incorporated into various systems such as a domestic hot-water supply system. To meet this demand, a pulsating combustion system capable of considerably varying the combustion power. However, the variable width of performance of the pulsating combustion system cannot be controlled freely in terms of the fact that air is self-attracted. Namely, when the combustion amount is increased or decreased, shortage or surplus of air results, and therefore, the combustion amount can be varied merely in the extremely narrow range.
The aerodynamic valve used in the pulsating combustion system as mentioned above cannot completely prevent a backflow. Therefore, a part of the combustion gas flows back into the air-intake chamber, and the backflow combustion gas is again taken into the combustion chamber. As a result, it becomes difficult to self-attract air necessary for combustion, often failing to obtain a stabilized pulse combustion. In addition, a temperature of the mixture gas of air and fuel flowing into the combustion chamber when the operation starts is the same as that of atmosphere, but if the mixture gas is once fired, the temperature thereof rapidly rises. The temperature of the gas within the combustion chamber before and after firing at the time of starting operation rises from temperature T.sub.1 (=293K) to temperature T.sub.2 (=1573K). Accordingly, let V.sub.1 be the volume of the gas flowing into the combustion chamber prior to stating of operation, the volume V.sub.2 under the condition of temperature T.sub.2 is given by the following formula: EQU V.sub.2 /V.sub.1 =T.sub.1 /T.sub.2
The ratio of volume V.sub.2 /V.sub.1 before and after firing at the time of starting operation is approximately 5.7. Since a diameter of the tail pipe is constant, the flow velocity v of the combustion gas flowing into the tail pipe is proportional to the increase .DELTA.V in volume (v.varies..DELTA.V). The pressure loss .DELTA.P in the combustion chamber at that time is proportional to a square of the flow velocity v of the combustion gas (.DELTA.P.varies.v.sup.2). Accordingly, when the temperature of the combustion chamber before and after firing at the time of starting operation changes from 20.degree. C. (293K) to 1300.degree. C. (1573K), the pressure loss is about 29 times of that prior to firing.
Particularly in the case where the aerodynamic valve or tail pipe is provided, the pressure loss of the valve is great, and therefore, the pressure loss as a whole further increases. For fuel supplied to the pulsating combustor, natural gas is normally used. As an example in case of methane gas, when the concentration thereof in the mixture gas is 5% to 15%, good combustion is obtained. However, when the pressure loss increases after firing, air in the amount required for combustion cannot be obtained, and the concentration of fuel in the mixture gas becomes higher than 15%, giving rise to a problem that the mixture gas cannot be fired smoothly.
Recently, the pulsating combustion system having a pair of pulsating combustors parallel-connected has been used in order to cope with the problem of noises. In the pulsating combustion system using the pair of pulsating combustors, the gas intake, combustion-explosion, and gas exhaustion in the first pulsating combustor can be 180.degree. out of phase with those taking place in the second pulsating combustor. However, the aforesaid pulsating combustion system also has a problem similar to that of the pulsating combustion system using one pulsating combustor as previously mentioned.